1. Field of the Invention
The present invention relates to radio-frequency telecommunications and, more particularly, to a method and apparatus for frequency mixing of in-phase and quadrature components of wireless signals, such as satellite or cellular signals.
2. Description of the Related Art
Increasing market penetration of wireless-communication or, simply, “wireless” devices, such as cellular telephones, radios, global-navigation-satellite-system (GNSS) receivers and the like, is generally premised on reducing costs associated with deployment of such devices while maintaining or improving performance and/or adding features thereto. One way to accomplish this task is to continually improve upon the components that perform electronic functions for wireless communications. For example, through continuous improvement of such components, newer generations of such wireless devices are routinely deployed in smaller packages with more processing power and lower power consumption, yet cost less than their earlier counterparts.
One component of the wireless devices, which are vast in amount and varied in functionality, is a mixer. The mixer is typically implemented in one of two configurations. The first configuration is an up-conversion configuration, and the second configuration is a down-conversion configuration.
In the up-conversion configuration, the mixer “up converts” an input signal to a radio-frequency (RF) signal so as to include the information carried by the input signal. The up-converted RF signal may then be used to transmit such information over a wireless transmission medium. In the down-conversion configuration, the mixer “down converts” a transmitted RF signal to a baseband, near baseband or intermediate-frequency (IF) signal. The down-conversion of the RF signal allows for extraction of the information carried in such RF signal.
Often, there are other ways to build down converters without using IQ mixing. two (2) conventional mixers are needed to down convert an RF signal. Collectively, these conventional mixers are commonly referred to as “IQ mixers;” one for obtaining from the RF signal an in-phase signal (i.e., a “conventional I mixer”), and the other for obtaining from the RF signal a quadrature signal (i.e., a “conventional Q mixer”). The I mixer uses an output signal from a local oscillator (LO signal) to obtain the in-phase signal. The Q mixer uses the (or another) LO signal to obtain the quadrature signal. Typically, the in-phase LO signal is shifted 90° with respect to the quadrature phase LO signal.
The RF signal is split and applied to both the I and Q mixers, i.e., each mixer receives half the RF signal current. To obtain the in-phase and quadrature signals, the I and Q mixers mix the RF signal and the LO signal at each one-quarter interval of each cycle of the LO signal. The I mixer sequentially switches the RF signal to its output so that the output over the four one-quarter intervals (i.e., one cycle) of the LO signal follows a +I, +I, −I, −I sequence. Similarly, the Q mixer may sequentially switch the RF signal to its output so that the output over the four one-quarter intervals (i.e., one cycle) of the LO signal follows a +Q, −Q, −Q, +Q sequence.
The conventional IQ mixers or more particularly, the components thereof, are always active, even though only one (e.g., the I or the Q) of the conventional IQ mixers is operating at any given time. Thus, each of the conventional IQ mixers consumes power.
Moreover, noise on the output of each of the conventional IQ mixers is proportional to an amount of time that the RF signal is switched to the output. Thus, the noise on the output of each of the conventional IQ mixers is present over the entire cycle of the LO signal, and for a ring-type I/Q mixer, exhibits a signal-to-noise ratio of roughly about −3.9 decibels (dB).
In view of these challenges, there is a need in the art for method and apparatus for frequency mixing that provides improved noise and power consumption.